How do I remove weight from weight painting?
From the Manual on  blend mode:

Subtract: In this blend mode the specified weight is subtracted from the vertex weights. The strength determines which fraction of the weight gets removed per stroke. However the brush will not paint weight values below 0.0.

Lighten: In this blend mode the specified weight value is interpreted as the target weight very similar to the Mix Blend mode. But only weights below the target weight are affected. Weights above the target weight remain unchanged.

Blur: tries to smooth out the weighting of adjacent vertices. In this mode the Weight Value is ignored. The strength defines how effectively the blur is applied.